1. Field of the Invention
This invention relates to an improvement of an edge (periphery) of a nitride semiconductor single crystal circular wafer. The edge means a periphery of a wafer. An as-cut wafer has a sharp edge which causes break, crack, split, chip or scratch of the wafer. Then, the edge part is slantingly polished into a round periphery. The slanting polishing of edges is called “bevelling” or “chamfering” for discriminating the edge slanting polishing from flat polishing of wafer surfaces. Semiconductor nitrides mean gallium nitride (GaN), indium nitride (InN) and aluminum nitride (AlN). The nitrides are rigid but fragile materials. Processing of semiconductor nitride wafers is far more difficult than silicon (Si) wafers or gallium arsenide (GaAs) wafers.
This application claims the priority of Japanese Patent Application No. 2003-98979 filed on Apr. 2, 2003 and Japanese Patent Application No. 2003-275935 filed on Jul. 17, 2003, which are incorporated herein by reference.
It has been very difficult to produce a large good semiconductor nitride bulk single crystal wafer. Someone has recently succeeded in producing freestanding GaN wafers on a small scale. Most of the freestanding GaN wafers are still small rectangular plates whose side is about 10 mm to 20 mm. Technology of producing GaN wafers is still not fully matured for serving GaN crystals as a substrate wafer of making InGaN type blue light lasers on a mass-production scale. Circular single crystal wafers of InN and AlN have not been yet produced except experimental trials.
At length, the production of circular GaN single crystal wafers becomes feasible. An unprocessed sharp edge of an as-cut wafer causes cracks, scratches or breaks of the wafer. Peripheral parts (edges) of semiconductor wafers are used to be slantingly polished for avoiding occurrence of cracks or breaks. The process of polishing wafer peripheries into slanting edges is called “bevelling” or “chamfering”. Bevelling or chamfering has been one of established processes in the case of silicon wafers or gallium nitride wafers for which production technology has been fully matured. Silicon wafers are rigid, sturdy and tough. A silicon wafer is chamfered by bringing a periphery of the wafer into contact with a rotary whetstone, rotating both the whetstone and the wafer in reverse directions and polishing the periphery into a slanting, round edge. Toughness of silicon enables rotary whetstones to chamfer edges of silicon wafers.
2. Description of Related Art    {circumflex over (1)} Japanese Patent Laying Open No. 9-181021, “Beveling method of wafer”, proposed a method of bevelling a silicon wafer by a sophisticated diamond rotary whetstone including 5 wt. % to 30 wt. % of ultrafine diamond particles of a diameter from 3 nm to 18 nm and 70 wt. % to 95 wt. % of small sized diamond granules of a diameter from 5 μm (5000 nm) to 8 μm (8000 nm). This is a complex rotary whetstone including two different kinds of whetting granules, one is the ultrafine particles and the other is the small particles. {circumflex over (1)} wrote that the conventional rotary whetstones had been composed of gross granules of an average diameter of 15 μm to 30 μm for chamfering silicon wafers and the gross granules caused breaks of wafers or cracks on wafers. For avoiding the beaks and cracks, {circumflex over (1)} proposed silicon wafer bevelling by the fine particle whetstone. Use of the ultrafine particles of a diameter of 3 nm to 18 nm suppresses cracks or breaks from occurring. The small sized particles of a diameter of 5 μm to 8 μm were intentionally used for compensating for delay of chamfering induced by the ultrafine particles. {circumflex over (1)} was an improvement of bevelling silicon wafers by a fine particle implanted rotary whetstone for avoiding occurrence of breaks and cracks.    {circumflex over (2)} Japanese Patent Laying Open No. 6-315830, “Beveling method for cut-resistant material”, proposed an electrolytic bevelling method for silicon wafers. {circumflex over (2)} complained that silicon wafers were too rigid and resistant to mechanically bevel and diamond whetstones should be used for silicon bevelling which raised the cost of silicon chamfering. The electrolysis method of {circumflex over (2)} supplies silicon wafers with an electrolyte, applies voltage to the silicon wafer via the electrolyte and chamfers edges of the wafers by the action of electrolysis. {circumflex over (1)} and {circumflex over (2)} are improvements of silicon wafer bevelling.    {circumflex over (3)} Japanese Patent Laying Open No. 2002-356398 (Date of publication of application: Dec. 13, 2002), “Gallium nitride wafer”, proposed an invention of bevelling of GaN wafers by the same invention as the present invention.
Current chamfering technology of the inventors of the present invention has chamfered a circular GaN wafer by making use of a rotary metal-bonded circular whetstone having implanted diamond granules of #100 mesh to #400 mesh (optimum mesh; #200), circumscribing a circular wafer to the rotary whetstone, and rotating the rotary whetstone and the wafer in inverse directions at 800 m/min to 2000 m/min which is an ordinary range of rotation speed, as shown in FIG. 1. A periphery 6 of the circular wafer 2 is pushed to a concave rotary whetstone 3 which wears fixed diamond granules on an outer round surface. The wafer 2 is circumscribed with the rotary whetstone 3. A liquid is supplied to the rotary whetstone 3 and the wafer 2. The rotary whetstone 3 rotates at a high speed in a direction. The wafer is also rotated in an inverse direction at a low speed. The implanted granules abrade the periphery of the wafer at a high rate. The circumscribing contact applies strong shocks upon the edge of the wafer. It takes about ten minutes to twenty minutes to eliminate a diametrical 1 mm wide margin. It is rapid bevelling. A final shape of the edges is determined in accordance with the standard of the SEMI (Semiconductor Equipment and Materials International).
FIG. 5 depicts sizes of parts of a 2-inch GaN wafer in the chamfering steps. A necessary bevelling margin is a radial 1 mm width. An initial wafer has a 52 mm diameter and a 520 μm thickness. Before chamfering, an orientation flat OF and an identification flat IF are formed at peripheral portions by a dicer or a grinding whetstone for signifying crystallographical orientations. A final length of the OF should be 16 mm. A length of the IF should be 7 mm. The OF line should be cut along a line distanced by 2.32 mm from a point on the circumference. The OF is usually a cleavage plane {1-100}. The IF line should be cut along another line distanced by 1.25 mm at another point on the circumference. The IF is vertical to the OF. Relative positions of the OF and the IF are determined for clockwise aligning in the order when a facing surface is a top surface. Sizes of parts are predetermined as such.
Prior edge processing of the inventors of the present invention chamfers, for example, a 52 mmφ GaN wafer having OF and IF by circumscribing the wafer with a metal-bonded diamond rotating whetstone, rotating the diamond whetstone, grinding a circumference of the wafer till the outer diameter of the GaN wafer is reduced to 50 mmφ, as shown in FIG. 1. The diameter should be reduced by 2 mm. The periphery which is eliminated away is called a polishing margin. The polishing margin is 1 mm in radius or identically 2 mm in diameter. Hard diamond fixed granules have a strong force of abrading rugged wafer edges. Such bevelling for the diametrical 2 mm margin takes only about 20 minutes to 40 minutes. The processing time is short enough. The processing improves roughness of the wafer down to about Ra 10000 nm to Ra 6000 nm (Ra 10 μm to Ra 6 μm) in the best case. However, the yield of chamfering is low. Sometimes the periphery is scratched or chipped in the chamfering step. Sometimes the wafer cracks or breaks. Impulsive contact with the hard rotary whetstone is often destructive for the wafer. Ra 6 μm is the lowest limit in the circumscribing rotary contact whetstone. It is desired to reduce the roughness less than Ra 5 μm. But, such smoothness less than Ra 5 μm is beyond the power of the metal-bonded circumscribing rotary whetstone rotating at an ordinary speed. It may supposed that a very slow rotation whetstone will be able to bevel the fragile GaN wafer without scratching, chipping or cracking. A ten-hour whetting making use of the same metal-bonded diamond rotary whetstone succeeded in chamfering a circumference to roughness of about Ra 3 μm. However, the slow whetting has drawbacks of excess dissipation of whetstones, jamming of meshes with dust and big fluctuation of final properties of the wafer. The long-time whetting is not practical for giving mirror smoothness to GaN wafers.
Whetting powder of #200 is far coarser powder than the whetting powder for polishing the silicon wafers described in the previously cited {circumflex over (1)} Japanese Patent Laying Open No. 9-181021, “Beveling method of wafer”. Thus, a wafer polished by the #200 powder has a rough surface of Ra 10 μm to Ra 6 μm.
The inventors had employed the coarse whetting powder for bevelling GaN wafers. GaN wafers are rigid but fragile. The rigidity forced the inventors to employ the coarse powder. It is, however, difficult to finish GaN wafers due to the fragility. If a GaN wafer is in outer-contact with a rotating whetstone and the edge is bevelled by the rotating whetstone, the edge or the whole of the wafer breaks during the bevelling process with high frequency. Even if the edges are not broken, the chamfered edges suffers from high roughness of Ra=10 μm to 6 μm as described before.
It may be perhaps effective to use rotating whetstones of fine granules of smaller diameters for reducing the occurrence of scratches. However, employment of smaller-sized whetting granules requires longer time for polishing edges of wafers, which raises cost of edge polishing. Use of the fine granule whetstones has other drawbacks of raising the probability of cramming meshes with whetting wastes and shortening the lifetime of whetstones.
One purpose of the present invention is to provide a freestanding gallium nitride wafer which is free from occurrence of cracks from peripheral parts (edges).
Another purpose of the present invention is to provide a freestanding gallium nitride (GaN) wafer which is free from occurrence of scratches or breaks from edges.
Another purpose of the present invention is to provide a freestanding gallium nitride (GaN) wafer with clean edges which do not invite particle adhesion or waste contamination. A further purpose of the present invention is to provide a method of chamfering an edge of a nitride wafer without incurring scratching, splitting or breaking of the wafer. A further purpose of the present invention is to provide a method of chamfering an edge of a nitride wafer without clogging of meshes with whetting materials. This invention is also applicable to other nitride semiconductor wafers of aluminum nitride (AlN) or indium nitride (InN) besides GaN.